Mine was when I was playing S.T.A.L.K.E.R. Clear Sky.
I was searching for loots around Cordon, and eventually I came to a Loner camp which is under attack by some mutants. I helped them killing those mutants, and succeed. Well, their group was consist of 5 people, but 2 killed by mutants, so only 3 left. I decided to took a rest for a while, and sat around the fire. Then, I heard one of them started a conversation with Russian language, like he was expressing that he is tired or something. The other responded, seems like he agreed to his words. And then, the other one pulled out a guitar and starts to play a relaxing acoustic song.
That was the first time I'm impressed to S.T.A.L.K.E.R.'s atmosphere :smile: